To determine, with the greatest possible precision, the actual hearing ability of a patient, it is desirable to be able to specify that patient's response to a known sound pressure at the ear drum. Because the sound pressure at the ear drum is the pressure that drives the middle and inner ears, this pressure must be accurately known if the hearing deficit of an individual is to be accurately specified and a hearing aid for that individual properly calibrated. However, the sound pressure measured by a probe at a point within the ear canal away from the ear drum is not generally equal to the pressure which exists at the ear drum. The higher frequency components of a sound signal will produce standing wave patterns in the ear canal which result in a variation in the sound pressure measured by a probe tube as a function of distance of penetration of the probe into the ear canal. Consequently, existing probe tube sound pressure measuring techniques do not provide a reliable basis for estimating the actual sound pressures at the ear drum or for estimating the magnitude of possible errors if the measured ear canal sound pressure is assumed equal to the ear drum sound pressure. In addition, present techniques cannot reliably assure consistent measurements of the same individual from one test to the next since the location of the tip of the probe tube within the canal is not known with precision. Thus, probe tube tests which attempt to measure the hearing response of an individual at different times are not necessarily reliable because there is no assurance that the probe tube will be placed at the same position within the individual's ear each time that the test is taken.